Piston unit



Sept. s, 19m 4 J, F, C, @OSE Em 3,527,386

PISTON UNIT 2 Sheets-Sheet 1 Filed Oct.- 8. 1968 Sept. 8, 1970 J, F. c. cLosE ETAI- 3,527,335

` PIsToN UNIT Filed Oct. 8, 1968 2 Sheets-Sheet 2 United States Patent O 3,527,386 PISTON UNIT John F. C. Close, Lane Cove, and Roy A. Williams, Lugamo, New South Wales, Australia, assiguors to Unisearch Limited, Kensington, New South Wales, Australia, a company of New South Wales Filed Oct. 8, 1968, Ser. No. 765,832 Claims priority, application Australia, Oct. 19, 1967, 28,710/67 Int. Cl. F01b 13/06; G01f 1]/00 U.S. Cl. 222-50 9 Claims ABSTRACT F THE DISCLOSURE A drench gun for dosing animals is made substantially wholly of plastic, and for use with such a gun there is provided a composite piston unit that is largely made of plastic, that is effective, simple and inexpensive, that is readily adaptable to the manufacturing tolerances encountered in the production of plastic parts, and that is readily adjustable to compensate for wear of the plastic piston ring as a result of the reciprocating movement of the piston unit in the drench gun cylinder, so that the drench gun, with the improved piston unit, in addition to being relatively inexpensive and fully effective, `has a long, useful life.

SUMMARY OF THE INVENTION The piston unit that reciprocates in the cylindrical portion of a pistol grip type of drench gun has a tubular pist/on unit of hard plastic and specialized design through which the liquid material passes on its way from the storage container carried by the user of the gun to the gun nozzle through which the material is ejected in measured amounts. The piston unit also has a tabular sleeve of low friction plastic secured to the forward end of the piston rod, this sleeve being of specialized design and including a bell-like forward extension with a cylinder-engaging collar at its front. The piston yunit also has a diameter adjusting cone of relatively rigid material and of specialized design, adjustably screwed onto the piston rod and working through an expander tube of elastomeric material to exert outward pressure all around the inside of the cylindepengaging collar in an amount depending on the position to which the cone has been moved axially. Finally, the piston unit has an inlet valve inside the piston unit seating on the conical region where the interior diameter of the tubular piston rod changes from small to large, the biasing spring for the valve being positioned in the chamber forward of this conical region and having its forward end engaging a screw that has a bore through its center.

DESCRIPTION OF THE INVENTION The piston unit is described in further detail as a component part of a drench gun with reference to the accompanying drawings in which:

FIG. l is a view, in elevation, of the complete drench un; g FIG. 2 is a perspective view of the drench gun with an exploded ararngement of the piston unit components;

FIG. 3 is an enlarged fragmentary sectional elevation of an assembled piston unit and of a portion of the cylinder in which it operates;

3,527,386 Patented Sept. 8, 1970 FIG. 4 is an enlarged vertical elevation of the upper end of the movable hand grip portion of the drench gun viewed as indicated by the arrows 44 in FIG. 2; and

FIG. 5 is a sectional View of the parts shown in FIG. 4, taken along the line 5-5 of FIG. 4.

As illustrated in FIG. 1, the drench gun, generally de.

signated 11, has a nozzle 12 at the outer end of a nozzle 13 that is detachably secured by nOse piece 14 to the body 15 of the gun, this body 15 preferably being made of a substantially hard plastic that is opaque to light. The body 15 has a xed, hand grip portion 16 and a movable, hand grip portion 17, pivoted at 18 to the xed portion 16. The movement of the movable hand grip portion 17 relative to the fixed hand grip portion 16 actuates a composite piston unit 20, which is shown in greater detail in FIGS. 2 and 3. Connected to the rear of composite piston unit 20 is a exible supply tube 21 that extends to a container (not shown) that holds the liquid that is pumped by the drench gun 11 and discharged from its nozzle 12 into the mouth of the animal. The container is generally carried, like a knapsack, on the back of the person using the drench gun.

-Referring now to FIG. 2, the drench gun is there shown with the movable, hand grip portion 17 swung back on its pivot 18 considerably further than its normal, unsqueezed position relative to the fixed, hand grip portion 16. This is attained by suitably turning the knob 22 positioned at the end of a volume adjusting nut 23, which is threaded onto a volume adjusting screw 24. The forward end of this screw 24 is pivotally secured, by pin 29, to the iixed hand grip portion 16. The position of the volume adjusting nut 23 on the volume adjusting screw 24 determines the normal, unsqueezed position of the movable hand grip portion 17 of the gun relative to the xed, hand grip portion 16, to which normal position the parts are urged by a spring (not shown) positioned in the hand grip portions and held in place by the pivot pin 1-8.

In the swung back position of the movable hand grip portion 17 the composite piston unit 20 may then be moved rearwardly relative to the body 15 of the gun and thereby removed from the body 15. After having been so removed, the piston unit 20 is shown in FIG. 2 in exploded form.

Before describing this piston unit 20 in detail, reference is made to FIG. 3, which shows the assembled piston unit 20 in position in the cylindrical liner 25 of the cylindrical portion 26 of gun body 15 for one of the normal, unsqueezed positions of the hand grip portions 16 and 17. The interior of the cylindrical liner 25 forms the cylinder 27 of the drench gun. The lower rear portion of the liner 25 and of the cylindrical portion 26 is cut away in order to enable the upper end of the movable hand grip portion 17, which is bifurcated into two ears 28, to enter the cylinder 27 when pushing the piston unit 20 to its extreme forward or left-hand position (as the gun is viewed in these figures).

Turning now to the piston unit Ztl-which is shown in FIGS. 2 and 3-the piston unit comprises a tubular piston rod 30 made of a substantially hard plastic, e.g., Celcon or Delrin The forward portion 31 of the rod has a larger exterior diameter than the rearward portion 32, and the same is true for the interior dia-meters of the rod 30. At the junction 33 of these two portions of the rod, the rod is conically shaped so that the 3 interior of the cone in this area forms a seat for a hemispherical inlet valve 34.

The rearmost end of piston rod has an enlarged boss which is externally threaded. 'I'he supply tube 21 from the container is coupled to the piston rod 30 by being secured to this boss 35.

Positioned forward of boss 35 and on the small diameter portion 32 of the pison rod, is a rear guide Collar 36 which assists in maintaining the piston unit 20 in axial alignment in the cylinder 27 of the drench gun.

The ears 28 of the movable hand grip portion 17 t into the space between the rear guide collar 36 andboss 35, so that the ears engage the collar 36 when the hand grip portions 116, 17 are squeezed to move the piston unit forward, and the ears engage the boss 35 when the hand grip portions 16, 17 are released to move the piston unit rearward. The space between rear guide collar 36 and boss 35 is provided with one or more axial lugs 37 that are normally positioned at an angle to the inner facing surfaces of ears 28 and thereby prevent rotation of the piston rod 20 about its axis. Such rotation is particularly objectionable when the supply tube 21 is being connected to or disconnected from the boss 35 of the piston unit, by turning the supply tube fastening or union nut (not shown) that screws onto boss 35.

It is preferred to interpose a filling and compression pad 67 of semi-elastic material on the movable hand grip portion 17, between its ears 28 as shown in FIGS. 4 and 5 so that the pad 67 is beneath the part of the rearward portion 32 of piston rod 30 that is engaged by the ears 28. The pad 67 has a base 618 with lobes 69 that project in front of and in rear of the adjacent ears 28 and thus secure the pad 67 in place. The pad I67 also has a frusto-conical centerpiece 70 that projects upwardly and engages the piston rod 30 in the region of its lugs 37.

Piston unit 20 preferably has a second guide collar 38 positioned forward of guide collar 36, and preferably this is located at the rear of the large diameter portion 31 of the piston rod 30, where it is shown.

The bore of the large diameter portion 31 of piston rod 30 constitutes a chamber 40 through which passes the liquid to be administered by the drench gun.

The forward end ofthe larger diameter portion 31 of piston rod 30 is internally threaded, as at 41, for receiving a screw 42, preferably made of non-corroding metal, such as brass. This screw 42 has a radial flange 43 at its front that overlies the front end of the large diameter portion 31 of the piston rod and extends radially outward beyond the exterior surface of such portion 31 to overlie part of the shoulder 44 of a tubular sleeve 45 positioned on the large diameter portion 31 of piston rod 30. In this manner the screw 42 axially secures or locks the tubular sleeve 45 onto the piston rod 30, as the rear end of the sleeve 45 abutts a collar 46 formed around the exterior of the larger diameter portion 31 of piston rod 30.

This sleeve 45 is preferably made of a semi-flexible low friction plastic, eg., Teflom and friction fitted onto the piston rod 30. Also, it is preferably additionally secured in place by a metallic clamp ring 47 around the periphery of sleeve 45.

The tubular sleeve 45 has a tubular extension 48 of greater diameter than sleeve 45 and projecting forwardly of the shoulder 44. At the forward end of the extension 48 it has a ring collar 49 formed around its periphery, the collar having sharp corners 50 and 52 at the forward and rear ends of the exterior cylindrical surface 51 of the collar 49. The diameter of the exterior cylindrical surface 51 of the collar 49 isl normally slightly larger than the interior diameter of the cylindrical line1 25 of the gun.

The resulting construction is a bell-like extension 48 at the forward end of the piston unit 20, the extension having a collar 49 serving as a piston ring engaging the cylinder y27 (i.e., the interior surface of cylindrical liner 25) in an interference fit. The material of which the bell-like extension 48 is made (i.e., Teiion) is suiciently resilient, in

conjunction with the length of the bell-like extension 48 forwardly of the point of fastening the extension to the forward portion 31 of the piston rod (i.e., forwardly of the shoulder 44) that the piston unit 20 is able to enter, and slide back and forth in, the cylindrical liner 25. A chamfer 27a is provided at the mouth of the cylinder 27 to facilitate the entry of collar 49.

Inside the bell-like extension 48 is an expander tube 55 of elastomeric material and of such diameter that the exterior cylindrical surface of expander tube 55 engages the inner cylindrical surface of the extension 48, and is of such axial length as to extend from shoulder 44 forwardly to the forward end of extension 48. The radial thickness of expander tube S5 is such that its inside dia-meter is slightly larger than the diameter of ange 43 on screw 42, so that the locking screw 42 may be assembled onto, and removed from, the end of piston rod portion 31 without disturbing the expander tube 55.

The locking screw 42 has an axial bore through its center and this bore is threaded, as at 5'6.

A diameter adjusting cone 57, preferably made of substantially hard plastic, has a rearwardly extending tubular portion 58 that is externally threaded so as -to screw into the threads S6 of locking screw 42. Cone 57 has a radial, disc-like portion 59 connecting the tubular portion 58 with a frusto-conical portion -60 extending forwardly from the disc-like portion 59 at an angle from the radius greater than degrees. The smallest diameter of the exterior surface of the frusto-conical portion 60 is slightly less than the internal diameter of expander tube 55. The angle of inclination of the exterior surface of the frusto-conical portion 60 (110 degrees) is such that such exterior surface contacts the inside of expander tube 55 substantially before the diameter adjusting cone S7, in moving axially rearward as a result of rotating the cone `57 relative to the threads 56 in the locking screw 42, is moved to -its extreme rearward, screwed-in position. Hence, starting with the position of di-ameter adjusting cone 57 when the exterior surface of the frusto-conical portion 60 rst engages the inside of expander tube 55, further rotation of the cone S7 in the direction to move it rearward, causes outward pressure to be exerted on the inside of expander tube 55 all around the periphery of expander tube 55. This outward pressure is transmitted by expander tube S5 to the underside of bell-like extension 48 in the area of its coll-ar 49, and all around the periphery of collar 49. As a consequence, there is a tendency for the collar 49 to expand radially, all around its periphery. Thus, for every setting of the diameter adjusting cone 57 after its frusto-conical portion 60 first engages the inside of expander tube 55, this outward pressure maintains the collar 49 in engagement with the cylindrical liner 25 all around the periphery of collar 49 and throughout the axial movement of piston unit 20 in its normal operating range of movement in cylinder 27.

An efficient and effective seal is thus readily established between the piston unit 20 and the cylindrical liner 25 in spite of variations in diameter of the pertinent plastic parts due to necessary tolerances in their manufacture, this being effected by rotating diameter adjusting cone 57 to an appropriate position. Furthermore, as wear is experienced with use of the gunparticularly Wear of the collar 49-such wear can be readily compensated for by rotating diameter adjusting cone 57 to a new position.

To facilitate the rotation of diameter adjusting cone 57, its radial, disc-like portion 59 has two apertures 61 therein, into which a suitable two-pronged key can t. Such rotation of cone 57 is performed, of course, after the piston unit 20 is removed from the cylindrical liner 25 of the gun.

To facilitate the insertion and removal of locking screw 42, its ange 43 preferably has two radial slots formed therein at an angle of to each other and accessible to a suit-able tool applied to the forward end of the piston unit 20 before the diameter adjusting cone 57 is attached to the piston unit 20. To avoid unnecessary confusion in the drawing, these slots are not shown.

The frusto-conical portion 60 of the diameter adjusting cone 57 desirably has a lip 62 at its forward end. The exterior surface of lip 62 is at an angle from the radius `greater than that of the main body of the frusto-conical portion 60. This provides a means whereby, if necessary, a small axial movement of the diameter adjusting cone 57 near the end of its inward movement can achieve a signicant expansion of the collar 49 on the bell-like extension 48 of the piston unit 20.

The diameter -adjusting cone 57 has at its center, along the axis of threaded tubular portion 58, a through passageway 63. The dia-meter adjusting cone 57 also secures in position the forward end of helical spring `64 positioned in chamber 40 of the piston unit, this forward end of the spring abutting the rearward end of threaded tubular portion 58 of the cone 57. The rearward portion of helical spring 64 surrounds a pin-like forward extension 65 of the hemispherical inlet valve 34 (which is preferably made of resilient material e.g. neoprene), with the rearward end of the spring abutting the at forward face of valve 34. Hence the spring 64 biases the inlet valve 34 to closed position. The spring 64 and v-alve 34 are readily removed for cleaning the chamber 40, or for substituting a new valve, by merely unscrewing the diameter adjusting cone 57.

After the diameter adjusting cone 57 has been rotated to the position believed appropriate, the piston unit 20 is inserted into the cylindrical liner 25 of cylinder 27 and moved forward by hand to where the collar 49 engages the full periphery of the liner 25. (Note, as mentioned earlier, that for a distance forwardly from the rear end of liner 25, the lower part of this liner 25, and the lower part of cylindrical portion 26 of the gun body 15, are cut away to enable the ears 28 of the movable hand grip portion 17 to move the pi-ston unit 20 to its extreme forward position. Hence, when the piston unit 20 is rst inserted into the rear of cylindrical liner 25, and moved for a distance forward from the initial insertion position, the collar 49 does not engage the liner 25 around its entire periphery.)

With the piston unit 20 in position where its collar 49 engages liner 25 around its entire periphery, the hand grip portions 16 and 17 are squeezed (preferably by the left hand) to cause the movable hand grip portion 17 to rotate counterclockwse (as viewed in FIGS. 1 and 2) until its ears 28 engage the collar 36. The knob 22 is then turned (with the other hand) to move the volume adjusting nut 23 to a position that secures the hand grip portion 17 in the position in which it is being held by the first hand.

The hand grip portions 16, 17 may then be fully squeezed and released several times to test the setting of the diameter adjusting cone 57 of the piston unit 20. If, for example, the piston unit 20 moves too hard when the hand grip portions 16, 17 are squeezed, or returns to its starting position too slowly when the hand grip portions 16, 17 are released, then the diameter adjusting cone 57 should be unscrewed a bit to diminish the outward pressure on the inside of collar 49 of the bell-like extension y48 of the piston unit 20.

The cylindrical portion 26 of the drench gun body 15 has an axially elongated window 71 along each side of the stationary hand grip portion 16. The cylindrical liner 25 is preferably made of a transparent plastic. Hence, the forward end of the piston unit 20 is visible through the window 71. Along the window 71 are indicia marks 72 with labels, such as the letters, A, B, C and D (or actual numbers) indicating the volume of liquid discharged by the gun by a full squeeze of the gun from a normal or starting position where the front of the piston unit (i.e., the collar 49) is opposite such indicia mark. The user of the gun can readily set the piston unit to eject the Cil desired amount of liquid for each squeeze of the gun, by turning the knob 22 of the volume adjusting nut 23 While the gun remains unsqueezed, at the same time observing through window 71 the resulting position of the front of the piston unit in the gun cylinder 27, and checking that position against the indicia marks 72.

Of course, the smallest volume ejected per squeeze of the gun is when the front end of the piston unit is normally opposite the mark 72 indicated by the letter A, and the largest volume when normally opposite the letter D.

The nozzle 12 has an outlet valve (not shown) which is biased closed by the action of a spring, and which is opened when the pressure in the nozzle tube 13 exceeds a xed, relatively small value.

In preparation for operation, the proper setting of the diameter adjusting cone '57 of the piston unit 20 is ascertained, in the manner described previously. The piston unit 20 is then left inserted in the cylinder 27 of the gun, and the exible supply tube 21 connected to the rear of the piston unit 20, if it is not already so connected. Then the initial, normal or unsqueezed position of the piston unit in the cylinder 27 is set by appropriate rotation of the knob 22 of the volume adjusting screw 23. Finally, the user squeezes the hand grip portions 16, 17 of the gun several times to prime the gun, i.e., to ll the parts of the gun with liquid from the container. 'I'he parts so filled are the supply tube 21, the rear portion 32 of tubular piston rod 30, the forward portion 31 of piston rod 30 (this forward portion constituting the chamber 40 of the piston unit 20), the through passageway `63 in the diameter adjusting cone 57, the part of cylinder 27 forward of piston unit 20, and the nozzle tube 13.

Thereafter, in operating the drench gun, the user squeezes the hand grip portions 16, 17 while directing the nozzle 12 into the mouth of the animal.

As the hand grip portion 17 moves inward relative to hand grip portion 16, and thus moves piston unit 20 forward (i.e., to the left, as viewed in the figures), the pressure in the liquid in cylinder 27 forward of the collar `49 increases, as the inlet valve 34 is maintained closed by its spring 64. The pressure in cylinder 27 forward of collar 49 increases to the point when it causes the outlet valve in the nozzle 12 to open. Thereafter the liquid is ejected from the nozzle 12 as the piston unit 20 is moved forward to its extreme forward position.

The volume of liquid ejected is the volume displaced by the piston unit 20 in moving from its initial or normal position to its extreme forward position.

Note that as the piston unit 20 moves forward, with inlet valve 34 closed, the liquid in the rearward portion 32 of piston rod 30, and in the supply tube 21, does not move relative to the piston 20 or tube 21, although it does move relative to the gun body 15 and its cylinder 27 similarly as do piston unit 20, and supply tube 21.

Substantially at the time the piston unit 20 reaches its extreme forward position, the pressure in nozzle tube 13 falls to the point when the outlet Valve in the nozzle 12 closes.

Thus, when the user of the gun now relaxes his pressure on the hand grip portions 16, 17, the spring in the base of these hand grip portions causes the movable hand grip portion 17 to rotate clockwise relative to hand grip portion 16, -and thereby move the piston unit 20l rearwardly toward its initial or normal position. From the start of this return movement of piston unit 20, the hydrostatic pressure of the liquid forward of inlet valve 34 i.e., in chamber 40) is less than that to the rear of this valve (i.e., in the rearward portion 32 of tubular piston rod 30), so inlet valve 34 opens and liquid from the supply line 21 is caused to enter the chamber 40 and the part of cylinder 27 forward of piston unit 20 while the piston unit 20 returns to its initial or normal position.

When the piston unit 20 reaches its initial or normal position, the hydrostatic pressure of the liquid on the opposite sides of inlet valve 34 become substantially equal,

so the inlet valve 34 is closed by its spring 34, and the drench gun 11 is ready for another cycle of operation.

We claim:

1. A piston unit comprising a tubular piston rod hav ing a valve chamber in its bore, a valve seat in the valve chamber, an internal thread at one end of the bore, a spring loaded inlet valve urged against the valve seat, an internally and externally threaded nut entered into said internal thread in the bore; a piston comprising a mounting sleeve xed on said one end of the piston rod, said sleeve having a bell-end projecting outwardly from the sleeve, a ring collar on the outer end of the bell-end having an outside diameter which make it an interference t with respect to the bore of the cylinder in which the piston unit is to operate, an expander tube in the bell-end, a diameter adjusting cone for the said expander tube threaded into said nut, a passageway formed through said diameter adjusting cone, at least one piston rod centering guide collar on said piston rod and means on the piston rod engageable by piston actuating means.

2. In a drench gun for dosing animals with a liquid contained in a storage container, generally located on the back of the person using the gun, and wherein the drench gun has a spring loaded outlet valve at its nozzle, at the forward end of the gun, has a flexible tube connecting the storage container with the gun, has a plastic gun body with a cylindrical portion and a stationary hand grip portion below the cylindrical portion and immovable with respect thereto, has a plastic movable hand grip portion pivotally mounted at its bottom to the bottom of the stationary hand grip portion, to move in planes parallel to the axis of the cylindrical portion, with the top of the movable hand grip portion formed into two upward extending ears, one on either side of the axis of the cylindrical portion, and wherein the drench gun also has a spring acting between the two hand grip portions to urge them apart, and has an adjustable stop means for limiting the outward movement of the movable hand grip portion relative to the stationary hand grip portion, an improved piston unit for reciprocating in the cylindrical portion of such drench gun comprising:

(a) a tubular piston rod made of a substantially hard plastic, the forward portion thereof having a larger diameter, exterior and interior, respectively, than the rearward portion thereof, with a conically-shaped junction between these two portions, the rearmost end of the piston rod having an enlarged threaded boss to which is connected the flexible tube leading to the storage container, the small diameter portion of the piston rod having a first collar of cylindrical shape forward of the boss, with the ears on the movable hand grip portion interengaging the piston rod in the space on either side of its axis between the boss and the iirst collar, the piston rod having a second collar of cylindrical shape positioned forward of the first collar and near the rear of the large diameter portion and of sufficient diameter to guide the piston rod in the cylindrical portion of the drench gun, the piston rod also having a third collar forward of the second collar and smaller in outside diameter than the second collar, and the piston rod having its forward end internally threaded;

(b) a tubular sleeve of semi-flexible, low friction plastic fitted onto the forward end of the piston rod and held in place against the third collar by a screw fastening into the internal threads at the forward end of the piston rod and having a radial flange overlying part of a shoulder of the tubular sleeve, the tubular sleeve also having a tubular extension of greater diameter than the sleeve and projecting forwardly of the shoulder, with a ring collar around the periphery of the tubular extension, the exterior diameter of the ring collar being normally slightly larger than the interior diameter of the cylindrical portion of the drench gun so that there is an interference fit as the piston unit is inserted in the 'cylindrical portion of the drench gun, the sleeve locking screw having an axial bore that is internally threaded;

(c) an expander tube of elastomeric material positioned radially within the tubular extension of the tubular sleeve and engaging the inside of such extension;

(d) a diameter adjusting cone of respectively rigid material, having a rearwardly extending tubular portion that is externally threaded so as to screw into the threads of the sleeve locking screw, having a radial, disc-like portion and having a frusto-conical portion extending forwardly from the disc-like portion at an angle from the radius greater than degrees, so that upon rotating the diameter adjusting cone to move it rearward relative to the piston rod, the periphery of the cone causes outward pressure to be exerted all around the inside of the expander tube, and this causes outward pressure to be exerted al1 around the inside of the collar, thereby tending to cause the exterior of collar to expand radially, all around its periphery, whereby the fit of the collar in the cylindrical portion of the drench gun is readily adjustable to accommodate manufacturing tolerances and wear of the collar and of the cylin drical portion of the drench gun, the tubular portion of the diameter adjusting cone having a through passageway providing communication between the chamber of the piston unit within the forward portion of the piston rod and the cylindrical portions of the drench gun forward of the piston unit;

(e) an inlet valve of generally hemispherical shape positioned so that its hemispherical surface seats in the conically shaped junction between the small diameter and large diameter portions of the piston rods; and

(f) a spring within the chamber having its forward end butting against one of the partial closures of the forward end of the chamber and its rear end butting against the inlet valve to bias the inlet valve to closed position.

3. The invention as set forth in claim 2, in which the over-all diameter of the inlet valve and of the inlet valve spring is larger than the diameter of the through passageway but smaller than the diameter of the threaded bore of the sleeve locking Screw, so that the inlet valve and its spring may be removed and replaced upon unscrewng the diameter adjusting cone but without unscrewing the sleeve locking screw.

4. The invention as set forth in claim 2, in which the diameter adjusting cone is made of a substantially hard plastic, and in which the valve spring is helical with a diameter less than the outside diameter of the rearwardly extending tubular portion but greater than the diameter of the through passageway therein.

5. The invention as set forth in claim 2, in which the ring collar around the periphery of the tubular extension has sharp corners.

6. The invention as set forth in claim 2, in which the frusto-conical portion of the diameter adjusting cone has a lip at its forward end at an angle from the radius greater than that of the main body of the frusto-conical portion.

7. The invention as set forth in claim 2, in which the tubular sleeve is additionally secured in place by a metallic ring around the outside of the sleeve just rearwardly of the tubular extension.

8. The invention as set forth in claim 2, in which the cylindrical portion of the gun body is opaque and has a transparent plastic cylindrical liner in which the piston unit reciprocates, the opaque cylindrical portion having an axially elongated window through which, and through the adjacent transparent liner, the forward end of the 9 piston unit is visible, so that the initial position of the piston unit may be readily observed as its initial position is changed by adjusting the stop means that limits the outward movement of the movable hand grip portion relative to the stationary hand grip portion.

9. The invention as set forth in claim 2, in which the rearward portion of the piston rod between the enlarged boss and the irst collar has at least one axial lug on its exterior that engages the adjacent ear of the movable hand grip portion and thereby prevents significant rota- 10 tion of the piston unit.

References Cited UNITED STATES PATENTS 2,574,109 11/1951 Kane, et al. 92--206 3,197,070 7/ 1965 Pearl et al. Z22-79 ROBERT B. REEVES, Primary Examiner H. S. LANE, Assistant Examiner 

